config: Add 'config_file' parameter to ConfigFileFinder

The `--config` flag is passed into `MergedConfigParser.parse()` and the
module-level function `config.get_local_plugins()`.  Since both of these
places utilize the `ConfigFileFinder` object and the configuration file
override pertains to how configuration behaves, this incremental change
directly associates the `ConfigFileFinder` and the configuration file
override.